Background: A specific inspiratory muscle ‘warm-up’ (IWU) prior to assessment of maximal inspiratory mouth pressure (PImax) may reduce the number of measurements required to obtain reproducible, representative estimates of PImax. The influence of inspiratory muscle training (IMT) upon this phenomenon is unknown. Objective: Compare the impact of an IWU on the between- and within-day reliability of PImax before and after IMT. Method: Eight participants were assessed on 4 separate occasions: 2 trials preceded IMT and 2 followed it. At each assessment, the highest of 3 initial efforts was recorded as the pre-IWU value (PI). The highest of 9 subsequent efforts that followed 2 sets of 30 breaths at 40% PI was recorded as PImax. Following 4 weeks of IMT, the trials were repeated. Results: IWU increased PI by 11–17% (p ≤ 0.01), irrespective of IMT status. After IWU, 5–6 efforts were required to determine PImax, irrespective of IMT status. PImax was similar between the 2 trials before IMT and the 2 trials after IMT (p ≥ 0.05), and was 21% higher after IMT (p ≤ 0.01). The coefficient of variation was excellent before and after IWU, both before (1.9 and 0.6%, respectively) and after IMT (1.1 and 0.3%, respectively). Limits of agreement and sample sizes for effect sizes ≤10% were substantially smaller after IWU in all trials. Conclusions: (1) IWU enhances the between-day reliability of PImax measurement, and this is unaffected by IMT, and (2) judgements regarding acceptability in relation to PImax reliability should be made in relation to analytical goals and we present data to facilitate this.